The research project is an investigation of longitudinal phonological development in three subject populations: normal (N equals 5), hearing-impaired (N equals 9) and Down's syndrome (N equals 8) children. The study is based on three independent, but related, sets of audio recordings - pre-speech vocalizations, elicited single word utterances (items from the Photo Articulation Test), and conversational speech. The data were collected at three-month intervals over a period of four years. The recordings are phonetically transcribed and then coded for storage and retrieval via computer. The pre-speech data are analyzed in terms of proportional frequency of sound segments and phonetic features for each child at each data collection session. The results are then compiled to reveal patterns of individual and group development over time. The meaningful speech data are analyzed with reference to the adult model word, to determine correct and incorrect productions. Error patterns are then described primarily as substitutions and phonological processes. As with the pre-speech samples, the data are analyzed longitudinally on both the individual and group level.